Introduction
The main goal of this lab is to gain experience on the measurement and interpretation of spectral reflectance signatures of various Earth surface materials captured by satellite images. This lab will teach how to collect spectral signatures, graph them, and conclude on what the curves on the graphs mean. The graphs will show reluctance on the y axis and the band number of the x axis.
Methods
12 surface materials form the image eau_claire_2000 were measured and plotted the spectral reflectance
1. Standing Water 2. Moving Water 3. Vegetation 4. Riparian vegetation 5. Crops 6. Urban Grass 7. Dry Soil 8. Moist Soil 9. Rock 10. Asphalt highway 11. Airport highway 12. Concrete surface.
First Erdas Imagine was opened and the image eau_claire_2000 was opened and I zoomed into the Eau Claire county and Chippewa area. Then home and drawing shown by arrows A and B below to activate area of interest tools.
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| Figure 1: A- Drawing B-Polygon tool |
First to collect standing water a polygon was drawn in the middle of Lake Wissota, this was done by clicking somewhere inside Lake Wissota and drawing any shape of polygon and double clicking to finish the circle. After the polygon is complete then next step is to click Raster followed by Supervised and Signature Editor shown by arrows A and B below.
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| Figure 2: A- Supervised B-Signature Editor |
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| Figure 3: Signature Editor. A- create new signature from AOI B- display mean plot window. |
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| Figure 4: Signature Mean Plot: the results I got from collecting a polygon from Lake Wissota. |
The next step is to collect and draw polygons on the surface materials 2-12 listed above. When collecting the surface materials I brought up Google Earth to help me determine what features are what, because it is very difficult to see where farms and vegetation exist. To display all materials 1-12 on one signature mean plot you can click, arrow B, shown in figure 5 below. Arrow A will scale chart to fit current signatures, which was used often to fit the curve on the graph.
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| Figure 5: A- scale chart to fit current signatures B- show all signature featurs |
One final step to help see the colors is to change the background of the graph, this can be done by clicking: edit>chart options>Plot background. I recommend changing the background to white. Also changing materials 1-12 to different colors is important, this can be done by clicking the color on figure 3. and choosing a color that is open.
Results
the standing water signature.
Band 1= highest, Bands 4 and 6 = Lowest
Why did standing water demonstrate the highest and lowest reflectance at the spectral channel (bands) you specified in Q.1 above?
Standing Water doesn't absorb band 1 well and also 2 and 3 (blue, green, red), therefore it deflects it making the color contain a lot of blue. While on the other hand reflectance is almost non-existent in the NIR, 4,5, and 6.
Specify the spectral channel (band) in micrometer for the highest and lowest reflectance for
signatures 2 through 12.
Sig 2- high 1, low 6
Sig 3- high 4, low 3 & 6
Sig 4- high 1, low 6
Sig 5- high 5, low 4
Sig 6- high 4, low 3
Sig 7- high 5, low 4
Sig 8- high 1, low 6
Sig 9- high 4, low 3
Sig 10- high 5, low 4
Sig 11- high 5, low 4
Sig 12- high 1, low 4
All graphs below are the results I obtained after drawing polygons of each surface material.
Why did vegetation display the highest and lowest reflectance at the spectral channels you
specified in Q3 above?
The reason band 4 is the highest is because
bands 1-3 are absorbed by the chlorophyll for photosynthesis. Also
plants deflect NIR, 4-6, to avoid destruction of protein cells.
At which spectral channel (band) does dry and moist soil vary the most? Explain reasons
for the differences.
Wet soil has a low reflectance in band
1 vs dry soil having very high reflectance in band 1. Overall dry soil has a higher reflectance
than wet soil, it is hard to see by looking at the graph but when looking at
the numbers it easy to see the difference.
This is because dry things reflect more than wet things and wet things
absorb more than dry things causing the difference.
Above is the final product with all 12 surface materials on one mean plot.
Describe spectral signatures that are most similar across the spectral channels and those that
greatly differ. Make sure you identify the surface features you are talking about. In your
response, outline reasons for similarities and differences of these spectral signatures across the
spectral channels.
Both Standing and Moving water follow
very similar paths if not exact.
Riparian vegetation and wet soil follow very similar paths. Crops and Dry Soil also follow very similar
paths. Airport, highway, and parking lot
all follow similar paths with medium reflectance. There seems to be a difference in wet vs dry,
wet being low reflectance and dry being high reflectance. As I stated before this is because dry
features reflect more than moist features.
If you are asked to develop a sensor that collects data for the identification of most of the
above surfaces, which specific spectral channels would you use and why?
When developing a sensor that
collects data for types of vegetation I would use bands 1-4 because chlorophyll
uses bands 1-3 for energy in the form of photosynthesis so we would see low reflect
and high absorption. As for bands 4,
plants reflect this to protect damage to proteins.
As for
water I would use bands 1-3 because it has a high reflectance compared to the
rest of the bands causing most of the water to appear blue.
Sources
All images were provided by Dr. Cyril Wilson of the Geography Department at the University of Wisconsin Eau-Claire.
